Comment by Ardinur

You can get loot from each rare spawn once per character per day, so you essentially have four chances to get the cage each day. Killing all four rare spawns that can drop the cage also gives you the achievement Hellbane.

Each cage you open has a 1/3 chance to give you one of the following mounts:

Comment by SerroDan

All 4 of the rares have the same minimum and maximum spawn timer this being between 1-3 hours.

When each of these rares spawn, a yell is put into the chat log and 3/4 of them will play a zone sound

When Terrorfist spawns no sound is played, but the Yell that is pasted into chat follows: "Frogan Yells: A massive gronnling is heading towards the Rangari Refuge! We are going to require some assistance!"

They don't however drop the mounts 100% of the time. In fact they don't even drop the mounts themselves, they actually drop a cage (Rattling Iron Cage) which drops the mount. The drop chance of this cage is 15% from all of the rares.

There are also 3 mounts to get from this cage! Each mount has an equal chance (1/3) of coming out of the cage. This being so you can get the same mount twice, three times, or even more! These mounts are: